For may years petroleum companies concentrated on developing oil and gas fields on land. But the world's appetite for energy sources, coupled with diminishing returns from land drilling, has driven petroleum companies to develop offshore reserves.
Sub-sea geologic sediments and structures are often similar and in some cases superior to geologic conditions that have proven highly productive on land. In fact, offshore reserves have been estimated at 21% of the world's proven reserves, with estimates that 40% to 50% of all future resources will come from offshore reserves.
A need exists for a method to store sufficient quantities of drilling fluids on a drilling vessel to reduce the dependency of a drilling operation on supplies brought in by work boats, thereby ensuring uninterrupted drilling in the event of inclement weather.
A further need exists for drilling fluid compositions suitable for storage on a drilling vessel.
Drilling offshore wells in deep water, greater than 1000 feet in depth, creates its own set of problems. When drilling on the edge of the continental shelf, quite frequently pressured shallow depth sands, of apparently artesian flow, are encountered. The depth of these sands and the pressures that they exhibit create a unique well design situation.
The unique well design is the result of being unable to hydrostatically control the shallow water flows (SWF) by the conventional method of returning the drilling fluid to the drilling rig. The hydrostatic head generated by returning the fluid to the rig exceeds the fracture gradients of the rock above the SWF. Therefore, the well is designed in a manner that a fluid of the proper density returns only to the sea floor, riserless drilling.
In a riserless drilling situation, large volumes of drilling fluid are required due to the fact that the fluid is not returned to the rig and reused. Depending upon the depth of SWF, volumes from 10,000 to over 30,000 bbls of drilling fluid could be required. The surface mixing equipment of existing rigs is insufficient to store or prepare the large volumes of fluids required to drill riserless. To date, riserless drilling operations have been dependent upon work boats and barges to store and transport the required fluids that were prepared at land based facilities. Often, bad weather has interrupted the supply of work boats and therefore the supply of drilling fluid, causing the termination of drilling operations.